We propose to develop radiligands suitable for idenfitying and quantifying receptors in human brain using PET. Ligands to be studied are those which bind to receptors implicated in the pathophysiology of affective disorders and parkinsons disease. These include ligands which bind to Alpha, Beta, seortonin-2 and dopamine-2 receptors. Previous studies examining in vivo binding of a number of ligands to rat, mouse or monkey brain show that in vivo measurements of binding of ligands to brain receptors is feasible. Such measurements in humans could provide a major breakthrough in the understanding and the treatment of both psychiatric and neurologic disorders. We propose to develop ligands which can be labelled with 18F for PET. In most cases this will involve the synthesis of new ligands. Each ligand will be investigated in vitro by standard techniques and those ligands which show high receptor affinity with a high degree of specificity will be further evaluated in studies in vivo using rats. Those ligands which show appropriate properties will be radiolabelled for studies in humans.